To date, most transistors used in power electronic applications have typically been fabricated with silicon (Si) semiconductor materials. Common transistor devices for power applications include Si CoolMOS, Si Power MOSFETs, and Si Insulated Gate Bipolar Transistors (IGBTs). While Si power devices are inexpensive, they suffer from a number of disadvantages, including relatively low switching speeds and high levels of electrical noise. More recently, silicon carbide (SiC) power devices have been considered due to their superior properties. III-Nitride (III-N) semiconductor devices are now emerging as an attractive candidate to carry large currents and support high voltages, and provide very low on resistance, high voltage device operation, and fast switching times. A typical III-N high electron mobility transistor (HEMT), shown in FIG. 1, comprises a substrate 10, a III-N channel layer 11, such as a layer of GaN, atop the substrate, and a III-N barrier layer 12, e.g., a layer of AlxGa1-xN, atop the III-N channel layer. A two-dimensional electron gas (2DEG) channel 19 is induced in the III-N channel layer 11 near the interface between the III-N channel layer 11 and the III-N barrier layer 12. Source and drain electrodes 14 and 15, respectively, form ohmic contacts to the 2DEG channel. Gate 16 modulates the portion of the 2DEG in the gate region, e.g., beneath gate 16.
In typical power switching applications for which high-voltage switching transistors are used, the transistor may be in one of two states. In the first state, which is commonly referred to as the “on state”, the voltage at the gate electrode relative to the source electrode is higher than the transistor threshold voltage, and substantial current flows through the transistor. In this state, the voltage difference between the source and drain is typically low, usually no more than a few volts, e.g., about 0.1-5 volts. In the second state, which is commonly referred to as the “off state”, the voltage at the gate electrode relative to the source electrode is lower than the transistor threshold voltage, and no substantial current flows through the transistor. In this second state, the voltage between the source and drain can range anywhere from about 0V to the value of the circuit high voltage supply, which in some cases can be as high as 100V, 300V, 600V, 1200V, 1700V, or higher. When the transistor is in the off state, it is said to be “blocking a voltage” between the source and drain. As used herein, “blocking a voltage” refers to the ability of a transistor, diode, device, or component to prevent significant current, e.g., current that is greater than 0.001 times the operating current during regular conduction, from flowing through the transistor, diode, device, or component when a voltage is applied across the transistor, diode, device, or component. In other words, while a transistor, diode, device, or component is blocking a voltage that is applied across it, the total current passing through the transistor, diode, device, or component will not be greater than 0.001 times the operating current during regular conduction.
When a device is operated in the off-state, large electric fields may be present in the material layers, especially when the device is a high-voltage device and is used in high-voltage applications. As used herein, a “high-voltage device”, such as a high-voltage transistor or diode, is an electronic device which is optimized for high-voltage switching applications. That is, in the case the device is a high-voltage transistor, when the transistor is off, it is capable of blocking high voltages, such as about 100V or higher, about 300V or higher, about 600V or higher, about 1200V or higher, or about 1700V or higher, and when the transistor is on, it has a sufficiently low on-resistance (RON) for the application in which it is used, i.e., it experiences sufficiently low conduction loss when a substantial current passes through the device. In the case the device is a high-voltage diode, when the diode is reverse biased, it is capable of blocking high voltages, such as about 100V or higher, about 300V or higher, about 600V or higher, about 1200V or higher, or about 1700V or higher, and when the diode is forward biased, it has a sufficiently low on-resistance RON or on-voltage VON for the application in which it is used. A high-voltage device may be at least capable of blocking a voltage equal to the high-voltage supply or the maximum voltage in the circuit for which it is used. A high-voltage device may be capable of blocking 100V, 300V, 600V, 1200V, 1700V, or other suitable blocking voltage required by the application. In other words, a high-voltage device may be designed to block any voltage between 0V and at least Vmax, where Vmax is the maximum voltage that could be supplied by the circuit or power supply. In some implementations, a high-voltage device can block any voltage between 0V and at least 2*Vmax.
Field plates are commonly used in high-voltage devices to shape the electric field in the high-field region of the device in such a way that reduces the peak electric field and increases the device breakdown voltage, thereby allowing for higher voltage operation. In a field-effect transistor (FET), the high-field region in the device is primarily in the access region between the gate and the drain, e.g., region 24 in FIG. 3. Hence, the field plate in a FET is typically placed on top of the portion of the drain access region adjacent to the drain-side edge of the gate, as seen in FIGS. 2 and 3. As used herein, the “access regions” of a transistor refer to the regions between the source and gate electrodes and between the gate and drain electrodes of the transistor, e.g., regions 23 and 24 indicated in FIG. 3. Region 23, the access region on the source side of the gate, is typically referred to as the source access region, and region 24, the access region on the drain side of the gate, is typically referred to as the drain access region. As used herein, the “gate region” of a transistor refers to the portion of the transistor between the two access regions, e.g., region 25 in FIG. 3.
Examples of field plated III-N HEMTs are shown in FIGS. 2 and 3. In addition to the layers included in the device of FIG. 1, the device in FIG. 2 includes a field plate 18 which is connected to gate 16, and an insulator layer 13 (e.g., a layer of SiN) that is at least partially between the field plate and the barrier layer 12. Field plate 18 can include or be formed of the same material as gate 16, or it can alternatively be formed of a different conducting material or layer. Insulator layer 13 can act as a surface passivation layer, preventing or suppressing voltage fluctuations at the surface of the III-N material adjacent to insulator layer 13. FIG. 3 shows an example of a III-N HEMT with a slant field plate. The device of FIG. 3 is similar to that of FIG. 2, except that the insulator layer 13 includes a slanted edge 26 on the drain side of the gate, and the field plate 28 is on top of and contacting the slanted edge 26; hence the field plate 28 is referred to as a slant field plate. The slanted edge 26 includes at least a substantial portion which is at a non-perpendicular angle to a main surface of the semiconductor material structure 32. Alternative field plate structures to those shown in FIGS. 2 and 3 have also been used.
In order for a field plate to effectively minimize the peak electric field when the device is blocking a voltage, it is electrically connected to a supply of mobile charge, which is typically accomplished by electrically connecting the field plate to the gate electrode, as shown in FIGS. 2-3, or in some cases by electrically connecting the field plate to the source electrode. As used herein, two or more contacts or other elements such as conductive layers or components are said to be “electrically connected” if they are connected by a material which is sufficiently conducting so that the electric potential at each of the contacts or other elements will be similar, e.g., about the same or substantially the same, after a period of time. Elements which are not electrically connected are said to be “electrically isolated”. Electrically isolated elements, although not maintained at substantially the same potential at all times, can be capacitively or inductively coupled.
While field plates have been shown to enable III-N HEMTs with very large breakdown voltages, they can cause an increase in the input capacitance (gate capacitance) of the transistor, resulting in slower transistor speeds and, in the case of power switching applications, larger gate currents during switching. In order to enable devices with even higher operating voltages and/or breakdown voltages than those which are currently possible with modern field plate structures, as well as improving other aspects of device performance, additional improvements in device design are necessary.